Tire Pressure?

Hey everyone - I have a question about what the proper inflation value is supposed to be for any given set of tires. Are you supposed to run at the maximum pressure stated on the sidewall or something less than that? I always thought that the max pressure was the proper inflation value, but every time I go into a tire store they always say my tires are overinflated. Are they stupid or am I? My max pressure on my tires is 51psi and they are always putting only 35psi in. It drives me nuts! What say you?

The numbers on the sidewall are maximums. BMW recommends 29 front and 32 rear in the book that came with my '87 325, so that's what I use. That seems to work OK for just normal around town driving, which is about all I do. It does look like the front tires wear a little faster at the outer edges, which indicates a little under-inflation, but it isn't bad.

QuoteJohn Yust
The numbers on the sidewall are maximums. BMW recommends 29 front and 32 rear in the book that came with my '87 325, so that's what I use. That seems to work OK for just normal around town driving, which is about all I do. It does look like the front tires wear a little faster at the outer edges, which indicates a little under-inflation, but it isn't bad.

John

Well then what is the purpose of buying a high performance tire? Generally the more expensive the rubber the higher the inflation pressures. If you are going to under inflate the tire why spend the money? I have always thought that higher inflation pressure keeps the tires tread more on the road surface when cornering hard etc. If I were to go around a corner at say 50mph and the front tires had only 29psi, I think the forces on the tire would deform it so bad you would be riding on the sidewall if not blow the bead and be flat. Then again, if I only inflated the tires to maximum pressure when I thought I was going to do some spirited driving then I would always be pissed cause my tires were low and I couldn't drift around my favorite turn. I am still confused!

Quotewodcutr
Hey everyone - I have a question about what the proper inflation value is supposed to be for any given set of tires. Are you supposed to run at the maximum pressure stated on the sidewall or something less than that? I always thought that the max pressure was the proper inflation value, but every time I go into a tire store they always say my tires are overinflated. Are they stupid or am I? My max pressure on my tires is 51psi and they are always putting only 35psi in. It drives me nuts! What say you?

I'm not an aggressive driver so I take the maximum pressure stated on the tires and the recommended pressure on the door sticker and average them. I don't differentiate between front and rear and so far I'm getting the best mileage for both fuel consumption and tire wear.

If I were concerned with ultimate handling, then my choices would likely be a lot different.

Quotewodcutr
Well then what is the purpose of buying a high performance tire? Generally the more expensive the rubber the higher the inflation pressures. If you are going to under inflate the tire why spend the money?

Let me preface this by saying I am not a tire expert. Heck, I'm not even a tire novice.
My guess would be that a high performance car does things that a normal car doesn't (I know, duh, but stick with me).
Those things that a normal car wouldn't do probably produces two scenarios:
1. The tire pressure increase. Heat, a hard bump, cornering, etc.
2. The car is in positions where a tire failure would be an incredibly negative experience, especially when compared to a normal driving flow out. A higher tires pressure rating might provide a higher level of insurance when abusing a tire.

But this is all fire side philosophizing, and is based on nothing but my immediate musings on the subject

~Tyler

ps
Third option, it's of some importance, but more a marketing thing than anything else.

If I were concerned with ultimate handling, then my choices would likely be a lot different.

Please elaborate!!! B)-

Ok...if I were to be doing auto-x, I might follow the advice of Ove Kvam and use tire pressures suited to my car, tires and the usage; if I were running on a track, I would probably take tire pressure advice from Rolf Christiansen.

What I pump my tires up to on the street and for maximum fuel economy would probably have the car handling like crap in those situations

OK. I'm late into the long fray here (long day of karting yesterday, woohoo! ), but I will jump in here. There is no one "perfect" tire pressure. The best tire pressure to use depends on both the kind of tire you're using and what your primary goal is: comfort? handling? fuel mileage?

That said, the maximum tire pressure written on the sidewall of the tire is just that: a maximum, above which you risk damaging the tire. 51 psi is very, very high -- much too high for any goal you might have. At high speed in hot weather your tires are probably nearing 60 psi, which is actually dangerous. When tires are overinflated the tread sort of balloons out, so that you are mostly riding on just the center portion of the tread. Not only does this give you significantly less grip, but your tires will wear out sooner than expected, and the center of the tread will be worn out while the edges are still good (because you're not riding on the edges).

The pressures recommended by the manufacturer and stamped on the doorframe are much closer to what you should be using. However, those pressures are almost always biased on the low side for a soft, comfortable ride. Many people, like me, are more interested in more grip around corners, so will increase those figures by a few psi. Generally I run about 32-35 psi all around. The higher side will give you slightly better handling and fuel mileage, while the lower side will be a little softer and more comfortable.

On the track, tire pressures make a huge difference in performance, and people spend a lot of time and effort tweaking pressures and figuring out exactly which pressures give the best lap times. For the tires I run on my track car (Toyo R888), I have learned that 40 psi hot is ideal. Lower than that, and the sidewalls flex too much and the steering response suffers. Higher than that, and I lose overall grip and my speed through the turns suffers. To reach 40 psi hot, I start out with 36 psi cold, and after several laps come into the pits and check the pressures immediately while the tires are still hot, and adjust as necessary to get to 40 all around.

It is very rare to find any situation anywhere where hot tire pressures above about 42 would be considered beneficial.

That being said, I run the lowest pressure that gives me max grip without 'rolling' over in tight corners at speed. That has given me 34 lbs on the front and 32 on the rears on Goodrich Traction T/As. generally, with the different tires that I have run on my E30, the pressures have run between 32-36 in front and 30-34 in the rear. Overinflating does not give more grip, just resistance to deformation and better mileage.

That being said, I run the lowest pressure that gives me max grip without 'rolling' over in tight corners at speed. That has given me 34 lbs on the front and 32 on the rears on Goodrich Traction T/As. generally, with the different tires that I have run on my E30, the pressures have run between 32-36 in front and 30-34 in the rear. Overinflating does not give more grip, just resistance to deformation and better mileage.

As Dave_G explained well, inflation pressure affects the shape of the tire contact patch and how the pressure across that contact patch is applied to the road. An under-inflated tire will put more load onto the shoulder edges of the contact patch, supporting the load more with the sidewalls of the tire, and will bulge the sidewalls out. An over-inflated tire will bulge the tire up, lifting the shoulders of the contact patch off the road, reducing the size of the contact patch and making the tire ride more along the centreline of the contact patch, less on the shoulders.

Ideally we would all use a pyrometer to measure our tire temperatures at the outer edge, centreline, and inner edge in order to always determine the optimum tire pressure to give an even tire temperature across the full width of the contact patch. An under-inflated tire will be hotter along the shoulders. An over-inflated tire will run hotter along the centreline of the patch. Run the tires long enough, either over or under-inflated, and eventually you'll be able to see either the centreline, or shoulders, wearing out faster.

Under-inflated tires will overheat faster and feel like mush because of the way they flex under side loads. Over-inflated tires will feel like they give crisper handling because they respond more quickly to steering inputs, but the smaller contact patch gives less grip particularly under braking. The optimum inflation pressure lies somewhere between those extremes and will depend on the type of driving you intend to do.

Quotewodcutr
Hey everyone - I have a question about what the proper inflation value is supposed to be for any given set of tires. Are you supposed to run at the maximum pressure stated on the sidewall or something less than that? I always thought that the max pressure was the proper inflation value, but every time I go into a tire store they always say my tires are overinflated. Are they stupid or am I? My max pressure on my tires is 51psi and they are always putting only 35psi in. It drives me nuts! What say you?

So you think max pressure is a good way to run your tires? thats funny!

So you think max pressure is a good way to run your tires? thats funny!

OK so I'm an idiot...I never had a father to tell me otherwise. I always thought that the pressure stamped in the side of the tire was the recommended pressure for that tire. I have now been given knowledge that I can pass onto my own son! I guess it's a good thing I don't buy high performance tires for all of my vehicles. They have mostly been stamped at 35 psi. :embarrassed:

So you think max pressure is a good way to run your tires? thats funny!

OK so I'm an idiot...I never had a father to tell me otherwise. I always thought that the pressure stamped in the side of the tire was the recommended pressure for that tire. I have now been given knowledge that I can pass onto my own son! I guess it's a good thing I don't buy high performance tires for all of my vehicles. They have mostly been stamped at 35 psi. :embarrassed:

Hey, I didn't mean to make you feel like an idiot but it was funny. There is a reason for door jam pressures...